Enargite is a scarce copper arsenosulfide mineral, Cu3AsS4. It has a metallic luster, dark gray to black color and streak, and a Mohs hardness of 3. This mineral often occurs in a massive or granular form, or in radiating masses. It can form tabular or prismatic or blocky crystals having a pseudohexagonal cross-section. Some crystal faces can have striations (see sample below). Enargite breaks along several planes of cleavage and also breaks along irregular fracture surfaces. It usually occurs in copper sulfide-bearing hydrothermal veins. It has been mined as an ore mineral of copper, but the arsenic is a "penalty" element". Extra money has to be spent during ore processing to remove the arsenic.
Enargite (2.2 cm across at its widest) from the Butte Mining District, southwestern Montana, USA. Enargite is one of the three most important copper-bearing ore minerals in the Central Zone of the Butte Mining District. For many decades, Butte was the # 1 producer of copper on Earth. Enargite at Butte occurs in copper sulfide-rich hydrothermal veins (dating to ~62-66 million years) that intrude the Butte Quartz Monzonite, a pluton of the Boulder Batholith (mid-Campanian Stage, late Late Cretaceous, 76 million years).